Partition of plasma hydrogen ion concentration changes during repeated sprints

Increases in blood [H+] and lactic acid [La-] attend fatigue. We applied Stewart's physiological model of acid-base status and simple regressions to assess the importance of independent variables and [La-] on [H+] during repeated sprints. Eight well-conditioned Arabians performed 9 sprints. Plasma from jugular vein samples was analysed for pH, PCO2, Na+, K+ and Cl-. Plasma [La-] was calculated from blood [La-], plasma [H+] from pH, SID from Na+, K+, Cl- and La-, Atot from pH, PCO2 and SID. Peaks for SID, PCO2 and [H+] were reached at sprint 1, -2 and -3, respectively. At sprint 3, the 5.7 nmol/l peak in [H+] was partitioned into 2.3, 2.7 and 0.7 nmol/l for Atot, PCO2 and SID, respectively. From sprint 3 to sprint 9, increases in Atot and decreases in SID tended to increase [H+] but were counteracted by a steady decrease in PCO2 that determined the progressive decrease in [H+]. Therefore PCO2 was the dominant determinant of [H+] during 9 repeated sprints, and the expected major effect of [La-] was moderated in the SID by opposing increases in [Na+] and [K+]. In the work-adapted phase (sprints 3-9), decreasing [H+] was correlated positively with PCO2 (r = 0.997, P < 0.001) but negatively with La- (r = -0.986, P < 0.001). Respiration was therefore completely compensating for the effects of metabolism on [H+]. During the transition from rest to sprint 3 (peak plasma [H+]), increasing [H+] was highly correlated (r = 0.99, P = 0.011) with [La-] but no other variable. The empirical and physiological analyses were consistent with one another during the work-adapted phase, but emphasis was placed on [La-] by the regression analysis, in contrast to PCO2 by the Stewart analysis, during the rest-work transition.